It is now well established that some DNA and some RNA-containing viruses, in addition to producing lytic infections in cultured cells, are capable of inducing tumors in selected animals. These tumor producing viruses have been shown to effect transformation of cultured cells through mechanism that probably mimic virus-caused transformation to malignancy in vivo. Details of the processes leading to malignant transformation are not established but the overall result is believed to involve the incorporation of viral DNA or - in the case of RNA-tumor viruses - a viral coded DNA into the chromosomal DNA of the host animal cell. The integration event and fixation of the transformed state may involve recombination between viral and host DNA. The structures required for and the molecular events involved in recombination in animal cells are not understood. Neither is there detailed understanding of the aberrant control which characterizes animal cells which have undergone malignant transformation. This first phase of our research is built around studies designed to advance our understanding of virus caused malignant transformation and the events involved in recombination between different viral DNAs and between tumor virus DNA and the chromosomal DNA of animal cells. These studies will focus upon the molecular parameters involved in human adenovirus-SV40 recombination and the genes in the parental and hybrid DNA molecules involved in malignant transformation.